Data from several laboratories have demonstrated elevated levels of endogenous digoxin-like immunoreactive factors (DLIF) in serum from patients with essential hypertension (EH) and from women with pregnancy induced hypertension (PIH). We have demonstrated that the adrenal gland secretes DLIF. Evidence also suggests that these factors are endogenous inhibitors of ouabain-sensitive sodium-potassium ATPase. Sodium-potassium ATPase is a modulator of vascular smooth muscle contraction and relaxation in arterioles. Inhibition of this ATPase enhances vascular smooth muscle contraction causing vasoconstriction which leads to systemic hypertension. Given these considerations it seems plausible that DLIF might be responsible for the vasoconstric- tion effects observed in these forms of hypertension and hence play a role in the presently unknown etiology of these diseases. We propose a working hypothesis: DLIF are endogenous inhibitors of ouabain-sensitive sodium-potassium ATPase and by this mechanism they increase blood pressure in EH and PIH. In this project we will test the first part of this working hypothesis. Demonstrating that DLIF from human adrenal glands is an inhibitor of ATPase requires purification and biological characterization of this endogenous factor. The aim of this project is to isolate, purify, and characterize the chemical nature and the biological activity of DLIF obtained from human adrenal glands. Characterization will be done utilizing four independent assays systems: radioimmunoassay, ouabain- displacement from ATPase, inhibition of sodium-potassium ATPase, and inhibition of rubidium-uptake in erythrocytes. To provide a more specific immunoassay for DLIF, antibodies against this factor will be developed. Because the binding of DLIF to serum proteins may regulate its bioactivity, the binding characteristics of this endogenous factor to serum proteins will also be studied. This research will provide the physical-biochemical and biological characterization of DLIF and test the hypothesis that DLIF are endogenous inhibitors of sodium-potassium ATPase. The general hypothesis proposing a link between endogenous digoxin-like immunoactivity, endogenous ATPase inhibitors, and EH or PIH can then be addressed in future studies. These endogenous compounds may prove useful in elucidating the mechanism of and/or provide a marker for these diseases.